1. Field of the Invention
This invention relates generally to the conversion of power from high to low voltage levels. In particular, this invention relates to the conversion of AC voltage to a relatively low level of DC voltage.
2. Description of the Prior Art
Most electronic devices operate from an AC power source of either 110 volts and 60 hertz, or 220 volts and 50 hertz. It is often the case that these electronic devices must in turn produce a constant low-level DC power. This is most often accomplished by an internal power supply which first converts the AC power to a high-level voltage which is subsequently converted to a lower level voltage.
The conversion from high-level DC voltage to low level DC voltage is often accomplished by applying the higher level DC voltage to the primary winding of a step-down transformer. The application of high-level DC voltage is moreover usually accomplished at a high frequency so as to cut down on the size and weight of the step-down transformer. Such a high frequency application to the relatively small transformer core can result in magnetic saturation unless the magnetic energy in the core is released by an opposite or cancelling flux. This is usually accomplished by closely regulating the high frequency application of the DC voltage level so as to allow for the subsequent cancellation of the magnetic flux build-up prior to saturation.
An internal power supply with the capability of applying a DC voltage to the primary winding at a relatively high frequency level while at the same time preventing magnetic saturation has been previously disclosed in the commonly assigned U.S. Pat. No. 3,924,172 to Gregorich. This particular internal power supply contains only one switching transistor employed for the purpose of selectively applying the DC voltage to the primary winding of the transformer so as to avoid magnetic saturation. This switching is often subjected to some relatively high voltage conditions immediately following a switch action. These voltage conditions are in large part attributable to the DC voltage applied to the primary winding of the transformer which is in turn a function of the standard AC power supply.